The present invention relates to cooling and lubricating the rotating shaft of a device, such as a centrifugal pump, which operates intermittently and which requires introduction of the lubricant and coolant only when the shaft is turning. In particular it relates to providing a simple cooling and lubricating system that is independent of the rest of the unit in which the centrifugal pump is incorporated. Most particularly it relates to the use of such devices on mobile units used in the oilfield.
In the case of a centrifugal pump stuffing box, oil must be supplied to the stuffing box for lubrication of seals and for cooling. In a classical solution for oil supply, a schematic of which is shown in FIG. 1 (prior art), a centrifugal pump [10], has a fluid suction line [12], a fluid discharge line [14], a stuffing box [16] containing a seal or seals (not shown), and a rotating shaft [18] driven by a motor [20]. The lubricating and cooling oil is provided from an external reservoir [22] to an external oil pump [24] that forces the oil into the stuffing box. It is necessary to have a shaft rotation detection device [26] so that lubricating/cooling oil can be provided from the external oil pump only when the shaft of the centrifugal pump is rotating. This device sends a signal to a controller [28] for the external oil pump so that the pump operates only when the shaft is turning. Otherwise, oil will be wasted, and undesirable contamination of the area around the centrifugal pump can occur, if oil is provided past a leaking seal, especially when the pump is not operating.
For common oilfield cement pumping equipment, the lubricant supply system of, for example, a typical centrifugal pump having a 6 inch nominal internal diameter inlet and a 5 inch nominal internal diameter outlet does not use an external oil pump as in the classical system shown in FIG. 1. Instead, as shown schematically in FIG. 2 (prior art), air pressure from an air compressor [30] forces oil from a supply vessel, such as an air-over-oil tank [32], into the stuffing box. As in the classical case, this air over oil lubricant/coolant supply system requires an external oil delivery control triggered by detection of the shaft rotation or engagement to the power source. On the lubricant line [34] that carries lubricant from the tank to the stuffing box, there are two valves. An air-actuated valve [36] completely opens or closes the line as the centrifugal pump power take-off (not shown) is engaged, thus forcing oil to flow into the stuffing box. The metering valve [38], downstream of the air-actuated valve, is set such that the correct amount of oil is delivered to the centrifugal pump when the pump is running. The metering valve is typically a needle valve but may be any suitable valve.
There are a number of problems with this system. The oil must be provided under pressure in this system, so an external compressor or pump is needed. The system requires control valves and metering valves that can fail, malfunction, or clog, so it cannot ensure that oil will be delivered when the shaft is turning. Also, it is complicated and is dependent upon many devices (such as motors, controls, detectors, monitors, and the like) that are extraneous to the system to be lubricated. If any of these components were to fail, lubrication/cooling would cease.
There is a need for a method in which the oil does not need to be provided under pressure, no external compressor or pump is needed, and no control valves or metering valves that can fail, malfunction, or clog are needed. There is a need for a method that ensures that oil will be delivered whenever, and only whenever, the shaft is turning, provided only that the lubricant source contains lubricant. The method should be very simple and independent of any devices (such as motors, controls, detectors, monitors, and the like) that are extraneous to the system to be lubricated.
A preferred embodiment is an apparatus, having a rotating shaft requiring a lubricating and cooling material to flow along the rotating shaft between the shaft and the seals while the shaft is rotating, that has an integrated axial flow pump that moves the lubricating/cooling material from an external supply vessel into the stuffing box through an inlet between the seals. Apparatus having such rotating shafts include, in particular, centrifugal pumps and vacuum pumps. Embodiments include using an integrated axial flow pump employing a blade or blades that are straight or spiral, or a groove or grooves that are straight or spiral. In another embodiment, the lubricating and cooling material is recirculated. In yet other embodiments, the integrated axial flow pump may be external to the stuffing box and lubricating and cooling material passes from that integrated axial flow pump to an inlet between the seals in the stuffing box through a conduit. In yet another embodiment, the integrated axial flow pump has at least one component integral to the rotating shaft and at least one component integral to the stuffing box. Other embodiments include methods of cooling and lubricating rotating shafts. Further embodiments include methods of pumping cement into a wellbore penetrating a subterranean formation with a pump cooled and lubricated with an integrated axial flow pump. The common feature is that the cooling and lubricating material is delivered to the appropriate sealing point on the rotating shaft whenever, and only whenever, the shaft is turning.